Vandaium-containing residues become available mainly in the form of used catalysts, e.g. from processes for the catalytic conversion of SO.sub.2 to SO.sub.3. The residues consist of the carrier material, generally silica, and the catalytically active substances.
In addition to vanadium compounds the residues may contain compounds of other metals, such as iron, arsenic, aluminum, calcium, cesium, and other alkali metals.
For ecological reasons such residues cannot be dumped or may be dumped only as hazardous waste because they contain water-soluble toxic substances.
For this reason and to recover valuable constituents of the residues, the residues are increasingly processed or worked up.
From Czechoslovakian Patent Specification 178,626 it is known to leach spent vanadium-containing catalysts in water with introduction of SO.sub.2 so that the vanadium compounds are dissolved as VOSO.sub.4. After the carrier material has been removed the VOSO.sub.4 in solution is oxidized to V.sup.5+ by the addition of MnO.sub.2 or NaClO.sub.3 and adjustment to a pH from 0.5 to 5.0. V.sup.5+ is extracted by an amine having a high molecular weight in a water-insoluble solvent and is then reextracted by an alkaline solution. Thereafter, V.sub.2 O.sub.5 is precipitated by a neutralization with H.sub.2 SO.sub.4. That process requires a large number of process steps and a consumption of relatively expensive chemicals and a solution which contains alkali metal sulfate is formed in large quantities. That solution poses problems in disposal or recovery.
U.S. Pat. No. 4,913,885 discloses for the processing of vanadium-containing raw materials and particularly of spent catalysts a process in which the material is leached in an aqueous suspension in the presence of mixed gases comprising SO.sub.2 and oxygen. It has been stated that conventional exhaust gases of industrial combustion processes which contain 5 to 20% SO.sub.2, 10 to 30% O.sub.2, balance inert gases, such as N.sub.2, are particularly desirable. As a result, vanadium as VOSO.sub.4 and other components as sulfates enter into solution. When the carrier material has been removed, the VOSO.sub.4 in the solution is oxidized by an addition of oxidizers, such as MnO.sub.2 or NaClO.sub.3 to V.sup.5+, which is then precipitated as NH.sub.4 VO.sub.3 by an addition of NH.sub.3, the precipitate is removed and the precipitate is then calcined to form V.sub.2 O.sub.5. The oxidation of VOSO.sub.4 to V.sup.5+ requires the addition of oxidizers and causes additional metals to enter into solution. The solution contains a number of dissolved metals and also surplus ammonium sulfates. This renders use of the solution, processing or disposal more difficult.